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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5586_Библиотеки_им_академика_М_И_Перельмана.pdf
Introduction to Pharmaceutical Biotechnology, Volume 2 (Second Edition)
evolution, dynamics and plasticity in the bacilli. B. megaterium is a nonpathogenic commercially existing host for the biotechnological production of
numerous substances such as vitamin B12, penicillin acylase and amylases. The
genome sizes of bacilli vary significantly, such as from 0.58 × 106 bp
(Mycoplasma genitalium) to 30 × 106 bp (Bacillus megaterium)[39, 41]. The
gene density is, however, comparatively constant at one gene per kilo base pairs.
• Vibrio cholerae O1 is reported as an important causative agent for infectious
diseases [41]. A number of bacteria like V. cholerae have two or more circular
chromosomes in their genomes. As a minimum, one of these chromosomes is
obtained from a plasmid.
• In the case of larger genomes such as E. coli, almost 50% of the ORFs (open
reading frames, the portion of a reading frame that has the prospect of being
translated) are of unidentified biological function [31].
• Around 25% of all ORFs are exclusive and do not have substantial sequence
resemblance to any other available protein sequence. Therefore, there are
some new protein families yet to be revealed. The incidence of paralogues
(pairs of genes that develop from similar ancestral genes) ranges from 26% for
M. genitalium (the smallest self-replicating organism and an effective human
pathogen related to a variety of genitourinary diseases) to 75% for
Pseudomonas aeruginosa. The incidence of paralogues increases with genome
mass [32].
• In close relation with bacteria, chromosomal transposals are more likely to
arise nearby the origin or terminus of replication. Several bacterial genomes
present in the database comprise phage DNA incorporated into the bacterial
chromosome. It is not uncommon for bacteria to hold several prophages in
their chromosomes, which then establish a substantial part of the total
bacterial DNA. Several prophages and prophage remnants litter many
genomes [33]
• Since severe complete genomes have been sequenced, gene order conservation
between diverse organisms is evolving as a revealing feature of genome [33].
Gene order conservation has been employed for forecasting the function and
functional interactions of proteins, as well as for investigating the developmental relationships between genomes [34]. The explanations for gene
order maintenance are still not well understood, since the association of the
prokaryote genome into operons and adjacent gene transfer cannot perhaps
be considered for all the examples of conservation found [45]. In fact, there is
very small preservation of gene order between phylogenetically distant
genomes. A statistically important preservation of gene order is most likely
indicative of the related genes being organized into operons. This characteristic has been employed to explore a number of new operons and to allocate
functions to the genes based on the prediction of operon function.
• Genes that are out of use for an organism are often called pseudogenes. In
general, inactivated gene copies are characterized by interference to their
reading frames due to frameshifts and premature stop codons. Pseudogenes
take place in prokaryotic genomes [35].
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In eukaryotes, the following features of genomes have been observed:
• The detection of orthologous groups is beneficial for genome annotation,
investigations on gene/protein evolution, comparative genomics and the
determination of taxonomically restricted sequences. The procedures effectively utilized for prokaryotic genome examination have, however, proved
difficult to apply to eukaryotes, since larger genomes can hold multiple
paralogous genes and sequence information is often inadequate. A great
number of the proteins encoded by the human genome have orthologues in
other eukaryotic genomes. For example, 60% of human proteins have
sequence resemblances to yeast, fly, nematode or plant proteins.
• Ancient transposon copies are found more in the human genome, whereas the
genomes of Arabidopsis, Drosophila and Caenorhabditis have transposons of
more recent origin [51, 52]. Bennett et al reported on comparisons between
Caenorhabditis (∼100 Mb) and Drosophila ( ∼175 Mb) using flow cytometry.
This work showed the genome size in Arabidopsis to be ∼157 Mb and
therefore ∼25% larger than the Arabidopsis Genome Initiative Estimate of
∼125 Mb [36].
• Transcription factors control the expression of genes at the transcriptional
level. Arabidopsis has not more than 20% transcription factors. These factors
are zinc-coordinating proteins [53]. In contrast, in the fruit fly, yeast and
nematode case, 51%–64% of the transcription factors are of this type.
• Arabidopsis transcription factors are represented by several genes and by the
assortment of gene families matched with those of D. melanogaster or C. elegans.
Around 50% of the genes recognized so far have unknown function [37].
• SINEs are retrotransposons that have appreciably significant reproductive
success throughout the course of mammalian development, and have played
an important role in shaping mammalian genomes [55]. Around 75% of the
repetitive DNA in human genome is because of LINEs (long interspersed
nuclear elements), these are sets of non-LTR retrotransposons [56] which are
extensive in the genome of several eukaryotes and in SINE (short interspersed
nuclear element) sequences. SINEs are sequences of non-coding DNA
existing at high incidences in numerous eukaryotic genomes.
• As the genome size increases, gene density declines. It ranges from one gene
per 2 kb in yeast to one gene per 10 kb in Drosophila, and one gene in 100 kb
in humans.
• The beginning of the genomic era has unlocked the doors to the study of
complete genome organization, not only in eukaryotes but also in prokaryotes, especially bacteria. Sufficient information on operon organization in E.
coli, together with the completed chromosomal sequence of this bacterium,
allowed the examination of the distances between genes and of functional
interactions of adjacent genes in the same operon, as opposed to nearby genes
in diverse transcription units. Bacterial genomes have distinguishing operons
(a section of DNA that a repressor binds to); the E. coli genome has 600
operons [38].
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• Approximately 15% of the ∼20 000 C. elegans genes are present in operons,
multigene clusters governed by a single promoter [39]. The C. elegans genome
contains operons similar to those of bacteria, and they contain 25% of its genes.
• Eukaryotic genomes have relatively large amounts of repetitive DNA; this is
a key reason for the variances in genome size.
• Transposable elements are found in the genomes of nearly all eukaryotes. The
human genome has a much higher density of transposable elements (44.4% of the
genome) than A. thaliana (10.5%), D. melanogaster (3.1%) or C. elegans (6.5%) [40].
• In plants such as maize and barley, genes are assembled in sections of DNA
that can further be divided by long stretches of intergenic DNA. Earlier
studies on the nuclear genomes of angiosperms showed that they are
distinguished by a compositional compartmentalization and that the vast
majority of genes derived from maize, rice, and barley are clustered in long
DNA stretches that are separated by vast expanses of gene-empty DNA.
• Several human ailment networks are highly preserved in the fruit fly.
Therefore, this insect can function as a model organism for the investigation
of even complex human diseases, such as neurological disorders. D. mela-
nogaster is a well-studied and highly tractable genetic model organism to
explore the molecular mechanisms of human diseases. Various biological,
physiological and neurological characteristics are maintained between mammals and D. melanogaster, and approximately 75% of human disease-causing
genes are assumed to have an efficient homolog in the fly.
• The incidence of sole genes (having no paralogues in the genome) is highest
(>70%) in yeast and fruit flies, and the smallest in Arabidopsis (35%).
• Variances in intron and exon structure are recognized in eukaryotic genomes;
the Arabidopsis genome varies considerably. It has been reported that there
are three basic patterns of exon–intron variation in eukaryotic genomes, and
these patterns were consistently present in almost all 13 genomes analyzed [41].
There is a better degree of ‘alternative splicing’ in humans than in the other
three eukaryotes. This facilitates more proteins to be encoded by each gene.
1. Various eukaryotic proteins are mosaic proteins, i.e., they are made of a
number of different domains. Around 90% of the domains known in
human proteins are present in Drosophila and C. elegans proteins.
Therefore, vertebrate development has been central for the creation of
the few new domains [
61]. Looking at both eukaryotes and prokaryotes,
we have learned the following: certain bacterial species have more genes
than lower eukaryotes.
2. Certain genes are present in a wide variety of organisms. For instance,
some of yeast genes have homologs among the human genes of
unknown function.
3. Unexpectedly, gene number does not essentially link with the complication of the organism or its evolutionary ladder.
Archaea is one of the three domains of life (alongside bacteria and eukaryotes). To
date, 16 complete archaeal genomes have been sequenced, with a conserved core of
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Introduction to Pharmaceutical Biotechnology, Volume 2 (Second Edition)
313 genes that are present in all sequenced archaeal genomes. The genomes of
Archaea have similarities to eukaryotic genomes. For example, certain genes look
like those of eukaryotes as they have histone genes and their DNA appears to be
prearranged into chromatin [42].
7.6.6 Gene order comparisons (for phylogenetic inference)
Comprehensive information on gene maps or even complete nucleotide sequences
for small genomes leads to the possibility of evolutionary interpretation based on the
macrostructure of complete genomes. The mathematical modeling of development
at the genomic level and the related inferential tools are, however, qualitatively
different from the normal sequence comparison theory established to study
evolution [43]. Equating gene order in dissimilar organisms is one of the techniques
for developing molecular phylogeny. Once the gene instruction in a given region of
the genome of two organisms is similar, they are called syntenic and the phenomenon is known as synteny. Gene order comparison in S. cerevisiae and C. albicans
discloses numerous cases of inversions, around half of which are single gene
inversions, in these two species separated by 140–330 million years [44].
Comparatively, inversions in prokaryotes tend to include much larger regions of
genomes and are focused on the origin and terminus of replication. Additionally, the
degree of inversion is much higher in eukaryotes than in prokaryotes. Furthermore,
the path of transcription appears to have little effect on gene location. A number of
regions of the human genome are highly conserved in dogs, cattle and sheep. One of
the main benefits of synteny is that information on gene location from a highly
mapped organism can be employed to locate the analogous gene in a poorly mapped
relative.
7.6.7 Phylogenetic footprinting (computational method)
Phylogenetic footprinting is a process for the detection of regulatory elements in a
set of orthologous regulatory regions from various species [45]. In other words,
phylogenetic footprinting is a method employed to detect transcription factor
binding sites (TFBS) inside a non-coding region of DNA of interest. This is done
by equating it to orthologous sequences in different species. Scientists have
discovered that non-coding pieces of DNA hold binding sites for regulatory proteins
that direct the (spatiotemporal) expression of genes. A general representation of
phylogenetic footprinting is shown in figure 7.8.
These TFBS or regulatory motifs are difficult to detect, mainly because they are
very small in length, and can display sequence variation. Two main concepts on
which phylogenetic footprinting rely are:
• Transcription factor functions and their DNA binding preferences are wellconserved between diverse species.
• Significant non-coding DNA sequences that are important for directing gene
expression will display differential selective pressure. A more gradual rate of
change occurs in TFBS than in others.
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Figure 7.8. Phylogenetic footprinting.
7.6.8 Origins, evolution and phenotypic impact of new genes
The genetic code is shared, and the organization of the codons in the standard codon
table is highly non-random [46]. The three main ideas on the origin and evolution of
the code are the following:
• Stereochemical theory: In this theory codon assignments are dictated by
physico-chemical affinity between amino acids and the cognate codons
(anticodons);
• Coevolution theory: This postulates that the code structure coevolved with
amino acid biosynthesis pathways.
• Error minimization theory: According to this theory selection to reduce the
adverse effect of point mutations and translation errors was the principal
factor of the code ’s evolution.
Thousands of genes present in eukaryotic genomes do not have complements in
prokaryotes. Several ‘new’ genes are mostly regular prokaryotic genes that have
been altered beyond recognition, such as domains involved in protein–protein
interactions. Several ‘new’ eukaryotic domains are α-helical; these could have
developed from the condensed coiled structures present in prokaryotes, which are
now particularly abundant in eukaryotes. Certain other genes have arisen in
completely unpredicted ways. For example, the hedgehog gene seems to have
developed by gene fusion; during development an intein domain has fused with an
extensively modified metalloprotease domain. However, the basis of several other
genes remains unknown. A number of new genes might have risen from transposable
elements. Among 13 799 human genes studied, 533 (4%) protein-coding regions
comprised of transposable elements or sections of such elements were found. Such a
transposable element could have incorporated into an exon. Otherwise, it may have
initially first incorporated into an intron and then enrolled as an exon. This can
become possible once the transposable element has possible splice sites. The
incorporation of transposable elements into eukaryotic genomes could be one
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reason for the high incidence of alternative joining in human protein-coding genes.
Therefore, incorporation of transposable elements into genes can produce new
genes. For example, in their protein-coding regions, two mouse genes have one or
numerous transposable elements, these genes have no human or rat orthologues.
7.6.9 The concept of minimum genome size
The ‘minimal genome’ method aims to estimate the smallest number of genetic
elements adequate to create a modern-type free-living cellular organism. Or, in other
words, the minimum genome size can be defined as the least number of genes
essential to sustain life, and can at best only be predicted. A simplified scheme of the
concept of minimum genome size is shown in figure 7.9.
For the following reasons, mycoplasma is considered a model organism for
minimum genome size:
• It is a member of the mollicutes.
• It evolved by massive genome reduction.
• It lacks genomic redundancy.
• It is an obligate parasite.
• It has the smallest identified genome of any free-living organism capable of
growing in axenic culture.
• It is wall-less.
Essential genes are considered important for its survival, e.g. genes that are encoding
for protein that maintains the central metabolism, replicate DNA, translate genes
into proteins, maintain a basic cellular structure, and mediate transport processes
into and out of the cell. Some essential genes can tolerate mutations that are
deleterious, but not wholly lethal, since they do not completely abolish the gene’s
function. Some of the essential genes, however, appear to perform non-essential
functions, such as aging and cell death, while many of the non-essential genes play
critical roles in cell survival. Essential genes can be difficult to study. If an essential
Figure 7.9. Concept of minimum genome size.
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Introduction to Pharmaceutical Biotechnology, Volume 2 (Second Edition)
gene is disrupted in the macronucleus, it is unlikely that complete replacement of the
wild-type copies with the selectable marker will ever be achieved. There are a
number of different transposon-based approaches available for defining essential
genes. There are two ways to identify essential genes or regions of the bacterial
chromosome which can be based on the location of the transposon insertion: the
‘negative’ and the ‘positive’ approaches. A problem in trying to identify essential
genes is that a knockout of an essential gene is lethal. Therefore, the use of the
negative approach will help with the identification of many regions that are not
essential and enable one to say with some certainty that regions in which transposon
insertions are not observed are likely to be essential.
Whole-genome sequences are now available for a great number of assorted
species [47]. Gene content quantification, expansion of the gene family, orthologous
gene conservation and gene displacement are promising for the assessment of the
minimal set of proteins adequate for cellular life.
The genes found in different organisms with the smallest genomes and their roles
are matched carefully. Moreover, investigational findings are collected by deactivating specific genes of these organisms and measuring the outcome on the organism’s
survival. Based on these investigations, it has been estimated that living creatures
need at least 250–350 genes. This minimum gene number is essential for the
organisms to exist as autonomous, self-replicating organisms. The unicellular
eukaryote minimum genome size was found to be 2.9 × 10
6
bp (the parasite
Encephalitozoan cuniculi). E. cuniculi infects different mammals, including humans,
and can be a source of digestive and nervous clinical syndromes in HIV-infected or
cyclosporine-treated people [48]. The genome of E. cuniculi has very little repetitive
DNA other than that for rDNA, and is expected to contain approximately 2000
genes [49]. Therefore, transformation from prokaryotes to eukaryotes has augmented the gene number by a factor of 7–8, however, the genome size has not grown
remarkably. The minimum gene number in multicellular eukaryotes, e.g.
Drosophila, Arabidopsis, etc, appears to be 16 000; this significant increase would
be essential for development and growth, and for sufficiently reacting to environmental or other external factors. Despite its toxicity, Fugu rubripes, the pufferfish is
widely consumed in Japan and considered the most delicious of all fish. Interest in F.
rubripes is not only restricted to taste and toxicity, however. The smallest genome
size for a vertebrate is that for the F. rubripes; it has only 4 × 10
8
bp DNA, but this
genome has 35 000 genes, which is similar to that of the human genome [50]. The
pufferfish genome has tightly packed genes, lacks repetitive factors and has single
short intergenic and intronic sequences. This drop in genome size is attained by
decreasing the sequences not involved in genetic functions to the minimum; it is
known as genome compaction. Otherwise, genome size may be decreased by gene
loss; this has happened in several parasites.
7.6.10 Comparative genomics analysis of mitochondria and chloroplasts
Comparative genomics investigation has offered new understanding of the origin of
organelles by endosymbiosis and exposed the vast evolutionary dynamics of
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Introduction to Pharmaceutical Biotechnology, Volume 2 (Second Edition)
organellar genomes. Moreover, they have significantly assisted in elucidating
phylogenetic relationships, particularly in algae and primary land plants with
inadequate morphological and anatomical diversity [51]. The genomes of mitochondria are often circular but linear molecules also occur. A number of observations on
mtDNA are as follows:
• Animal and fungal mitochondrial DNA (mtDNA) is very small (15–20 kb),
compared to plant mtDNA (200–2000 kb) which is more complex than that
of either animals or fungi.
• The greater genome size of plant mitochondria is mainly due to a greater
amount of spacer DNA. For example, Arabidopsis mtDNA is almost 20 times
larger than human mtDNA, but has less than twice the number of genes.
• The mtDNA of numerous species can be divided into two basic types:
ancestral and derived DNA. Plant and animal mtDNAs are derived genomes.
In the case of angiosperms, there has been extensive gene loss, however, the
mtDNA has grown in size because of the duplication and capture of DNA
from cpDNA and the nuclear genome.
• Mitochondria are thought to be derived from Rickettsia prowazekii, the
agent responsible for epidemic, louse-borne typhus in humans [75]. The
Rickettsia (alpha-proteobacteria) multiply in eukaryotic cells only. The
organization, structure and gene content of this bacterium look exactly like
the mtDNA of Reclinomonas americana [52].
• Seed plant mitochondrial genomes are exceptionally fluid in size, structure
and sequence content, with the accumulation and activity of repetitive
sequences underlying much of this variation. mtDNA genes have been
transferred into the nucleus, as this transfer has stopped in animals, however,
it continues to occur in the case of plants and protists [76]. For example, a
gene from mitochondria, cox2, is still under the development of transfer in the
case of legumes. In some legumes, cox2 is present in the mitochondria,
whereas in other organisms it is present inside the nuclear genome, and in
some it is present in both the nuclear and mitochondrial genomes [53].
• The three plant genomes gene types display differences in their evolutionary
rates. Nuclear genes develop the fastest, followed by chloroplast genes and
finally mitochondrial genes, in spite of the fact that the mitochondrial genome
displays extensive reorganization of its structural organization. This gradual
rate of evolution has made mitochondrial gene sequences unattractive to
plant phylogenetic studies at the suborder and subfamily levels, but has
proven to be valuable in inferring ancient phylogenetic relationships and in
approximating the time of early diversification events in seed plants. There
has been cross-species acquisition of DNA or horizontal gene transfer of
genes by plant mitochondrial genomes such as the homing group I intron,
introns that encode site-specific endonucleases to further catalyze their
effective distribution from introns including alleles to intron-lacking alleles
of similar gene in genetic crosses. This intron is present in the cox gene of
some angiosperm species [54].
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7.7 Gene estimation and counting
Annonation is a procedure which detects genes, and explores their regulatory
sequences and promising functions. Annonation designates the non-protein-coding
genes (those that do not encode protein sequences), e.g. coding genes for rRNA,
tRNA and nuclear RNAs, mobile genetic elements (DNA segments that encode
enzymes and other proteins that control the DNA movement inside genomes, called
intracellular mobility or, between bacterial cells, intercellular mobility) and repetitive sequence families existing in the genome. The role of annonation begins only
after the genome sequence has been attained. Gene estimation is a significant issue
for computational science. There are numerous logarithms that can be employed for
gene estimation by utilizing identified genes as a training data set which can be
further utilized to form a model or to test (or validate) the built model. Gene
counting is problematic until the actual locations of genes in a genome are
determined. The occurrence of overlapping genes and splice variations makes the
job more challenging as it becomes more difficult to forecast which region of DNA
should be considered as the same or as many different genes.
Numerous genes in eukaryotes have an arrangement of exons (coding regions)
trailed by introns (non-coding regions). Because of this, genes are not systematized
as continuous ORFs (open reading frames). An ORF has a series of codons that
require an amino acid sequence.
Also, eukaryotic genes are often widely spaced, thus there is a greater probability
of finding false genes. However, novel methods of ORF scanning software for
eukaryotic genes allow more effective scanning. It has been observed that approximately 99.8% of the 3.2 billion base pairs of two humans are identical and merely
0.2% are different. Of every 500 nucleotides, only one nucleotide varies between two
individuals. It means that a difference in a few sites in the DNA sequence can result
in severe disorders and dissimilar features in human beings.
7.7.1 Genome similarity, SNPs and comparative genomics
As discussed above, approximately 99.8% of every human genome is identical to any
other human genome and 0.2% different, which indicates that two individuals vary
only in 6 million locations out of 3.2 billion locations (figures 7.10 and 7.11). Two
human beings will be similar if these locations have little influence. As already
discussed, the human genome is closely related to that of chimpanzees (more than
98%). Therefore, variance in some locations in DNA results in a unique individual.
One nucleotide among every 500–1000 nucleotides always differs between two
individuals. The most considerable variations in individual genomes are SNPs,
which have been detected in both coding and non-coding regions of genome [55].
SNPs as discussed above are fundamentally varied in a single base, i.e., A, C, G or T
resulting in DNA variations which further result in the presence of different bases at
such locations. With varying individuals the type of nucleotide or base existing at a
given position on a chromosome can also vary. It has been observed that 90% of
sequence variation among humans is because of the presence of SNPs. Thus SNPs
offer a molecular marker that is present in high density. During DNA finger-printing
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Introduction to Pharmaceutical Biotechnology, Volume 2 (Second Edition)
Figure 7.10. An SNP map.
Figure 7.11. An SNP map to determine how patients are likely to respond to a particular drug.
in mainly non-coding parts of the genome, such genetic deviations among diverse
human beings are used. These genetic variations are also accountable for the severity
of disease and the reaction of the body to treatment.
7.8 Genomes: genome evolution
Genome trees are a way to contain the astounding amount of phylogenetic data that
is found in genomes. Diverse formalisms have been presented to construct genome
trees on the basis of various features of the genome [ 56]. On the basis of these
characteristics, we divide genome trees into five classes:
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